In this connection, it is known from U.S. Pat. No. 5973887 to estimate the NOx quantity, which is stored in a first phase, by a summed engine rpm and then to change into a second phase when the summed engine rpm exceeds a predetermined threshold value. The stored quantity is thereby modeled. The actual value of the stored quantity is not detected. The modeling can only incompletely simulate the actual stored quantity. For this reason, the first phase is either too long or too short with the above-mentioned disadvantageous consequences for the quality of the exhaust gas.
The U.S. Patent application Ser. No. 09/150,600 describes a possibility to fix the end of the second phase with rich engine operation, that is, oxygen deficiency in the exhaust gas. An oxygen-measuring probe in the exhaust gas rearward of the storage catalytic converter serves this purpose. As soon as the probe detects a crossover from a lean mixture to a rich mixture, this means that the storage catalytic converter no longer outputs sufficient oxygen for the oxidation of the hydrocarbons and the carbon monoxide and is empty of NOx containing oxygen (oxygen store and NOx store emptied). Thereupon, the second phase can be ended with oxygen deficiency and a reversal to the first phase with a lean mixture (oxygen excess) can be made. Thereby a breakthrough of CO and HC to end the regeneration phase is deliberately accepted after each storage interval. With each phase change, unwanted CO and HC emissions occur which can present problems for the maintenance of the toxic substance limit values in the sum over many phase changes. In addition, for each phase change, the oxygen store of the system is emptied and therefore fuel consumption is unnecessarily increased.